Pro-Trump, anti-Trump protesters clash in Berkeley, California

(CNN) At least 13 people were arrested as pro-Trump and anti-Trump protestors clashed Saturday at a park in Berkeley, California, police said.

"A large number of fights have occurred and numerous fireworks have been thrown in the crowds," Berkeley police said in a statement. "There have also been numerous reports of pepper spray being used in the crowd."

CNN affiliate KPIX reported that Trump supporters planned a "Patriot Day" rally at noon and counter-protesters showed up a few hours earlier.

Hundreds of people had gathered in Civic Center Park. Police set up a barrier of orange mesh fence to separate the two sides but it quickly fell down as protesters started fighting, KPIX said.

Video showed crowds spilling into nearby streets. Scuffles broke out, fireworks, bottles and traffic cones were thrown into crowds and dumpsters and trash cans were hauled into streets. One man set afire a red "USA" hat and held it overhead.

Police donned gas masks as they used pepper spray on the crowd. A Berkeley station for BART, the mass transit system, was shut down because of the disturbance, CNN affiliate KRON said.

Police said they confiscated prohibited items including hand-held flagpoles, a knife, a stun gun, helmets and signs, and flags attached to poles, KRON reported.

Video showed apparently injured people but no details on the number of injuries was available.

A police officer was treated and released from a hospital after someone threw either pepper spray or tear gas into the crowd, said Berkeley police spokesman Byron White. He added that a person in the crowd was treated and released after being sprayed with "bear spray," an irritant spray used to deter aggressive bears in wilderness areas.